This invention relates generally to solid state radiation imagers and in particular to structures in such imagers to reduce phantom noise and image artifacts.
Solid state radiation imagers typically comprise a large flat panel imaging device having a plurality of pixels arranged in rows and columns. Each pixel includes a photosensor, such as a photodiode, that is coupled via a switching transistor to two separate address lines, a scan line and a data line. In each row of pixels, each respective switching transistor (typically a thin film field effect transistor (FET)) is coupled to a common scan line through that transistor's gate electrode. In each column of pixels, the readout electrode of the transistor (e.g., the source electrode of the FET) is coupled to a data line. During nominal operation, radiation (such as an x-ray flux) is pulsed on and the x-rays passing through the subject being examined are incident on the imaging array. The radiation is incident on a scintillator material and the pixel photosensors measure (by way of change in the charge across the diode) the amount of light generated by x-ray interaction with the scintillator. Alternatively, the x-rays can directly generate electron-hole pairs in the photosensor (commonly called "direct detection"). The photosensor charge data are read out by sequentially enabling rows of pixels (by applying a signal to the scan line causing the switching transistors coupled to that scan line to become conductive), and reading the signal from the respective pixels thus enabled via respective data lines (the photodiode charge signal being coupled to the data line through the conductive switching transistor and associated readout electrode coupled to a data line). In this way a given pixel can be addressed though a combination of enabling a scan line coupled to the pixel and reading out at the data line coupled to the pixel.
The performance of flat panel imaging devices is degraded by capacitive coupling between data lines and the pixel photodiode electrodes. In particular, during some common imager operations the x-ray flux remains on during readout of the pixels. One example of such operations is fluoroscopy in small or less sophisticated units as might be used in surgery or portable applications; such units use light weight and low cost x-ray generators which must be on continuously to produce an adequate output signal. Such units further typically are not adapted to rapidly cycle the x-ray beam on and off during relevant periods to prevent radiating during the readout periods. Another example are imagers used in conjunction with radiation therapy in which the radiation source is on continuously (to maximize delivered dose) or is pulsed on periodically, which pulses can occur during the readout period. This simultaneous excitation of the imager while reading out pixels results in image artifacts or "phantom" images. The phantom images occur as a result of capacitive coupling between the respective photodiode electrodes and adjacent data lines; during the readout of a given pixel attached to a given data line, the potential of the other pixel electrodes (e.g., the non-read pixels) continue to change as the radiation flux strikes the imager. The change in potential of the pixels not being read out is capacitively coupled into the data line, thereby inducing an additional charge which is read out by the amplifier and presented as part of the signal from the addressed pixel. This effect produces cross-talk or contrast degradation in the image, and is commonly evidenced as bright lines in the display readout.
It is desirable that a solid state imager array exhibit minimal cross-talk and be capable of generating a stable and accurate image in multiple modes of operation, including modes in which pixels are being read out while the x-ray flux is being applied.